ALMOST FOUR million young Americans "voted" this summer on an issue that cuts closer to them than anything Americans as a whole will vote on in November: draft registration. The returns, based on both Post Office and Internal Revenue Service counts, are now in -- a General Accounting Office audit is in the works. They show that about 85 percent of the nation's 19- and 20-year-olds registered in the two chosen weeks and 6 percent have registered since, with more trickling in. Of these, 1.8 percent indicated they were conscientious objectors or were registering under protest. The government is now confirming the results by mail and phone, looking for non-registrants and studying the prosecution of violators.

The results show an impressive degree of respect for the law and/or for the burdens of citizenship among a slice of the population often regarded as being indifferent if not antagonistic to authority, war and personal risk. These are, after all, young people who grew up in a time when military service was dismissed as the preserve of a subculture. The anti-registration, anti-draft option was thoroughly publicized. Yet, though registration is meaningless unless you contemplate an eventual draft, most young Americans registered in a prompt and orderly way. Many blanks were filled in wrong, but Selective Service is quite sure the IRS culled out the "Donald Ducks."

President Carter, shying from linking registration to the political dread word "draft," asserts that the high turnout shows unity in the face of Moscow's invasion of Afghanistan. Who believes that? Gov. Reagan is still hung up on the contradiction between a sentimental rejection of a draft's coercive nature and his passion for national security. But most young Americans, as we read their message, were saying they are prepared to do their part.

Beyond that, the time it took for the two catch-up summer registration calls demonstrated that for Selective Service to furnish men to the military in a timely fashion in an emergency, registration must be continuous and cannot prudently be left until the time a president mobilizes. A system for continuous registration of 18-year-old men will take effect in January.

Important draft issues, notably the issue of women, remain on the national agenda. Within the more limited framework of registration, the discussion is now turning to peacetime classification -- tests to assess registrants' physical and mental suitability. It is a necessary question, but a technical one. The important question was posed by the summer calls: would young people give their informed consent to a sterner ethic of national service? They did.